Microwave switching device



Aug. 18., 1959 D. N. BREWSTER 2,900,568

7 MICROWAVE SWITCHING DEVICE Filed Feb. 17, 1958 Fig. l

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DUDLEY N. BREWSTER v ATTORNEY United States Patent MICROWAVE SWITCHING DEVICE Dudley N. Brewster, Wellesley Hills, Mass., assiguor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., acorporation of Delaware Application February 17, 1958, Serial No. 715,566

2 Claims. (Cl. 315-39) This invention relates in general to microwave switching devices and more particularly to an aperture or window for such devices.

It has been the practice for some years in the microwave art to control the flow of microwave energy, particularly in wave guides, by means of switch tubes, examples of which are the so-called TR and ATR tubes. These devices normally include at least one member which is resonant at substantially the frequency of operation, and an ionizable gas or other conducting medium is provided in the area of the resonant member. At predetermined levels of operation of the tubes, a discharge path is formed through the ionizable gas or other conducting medium in such a manner that the structure will no longer resonate at the frequency of operation, the discharge instead creating a short circuit across the wave guide section. In the absence of such discharge the resonant member provides an open cir' cuit. to switch energy at their points of occurrence or at a predetermined distance from the discharge.

In recent years the level of power at which microwave apparatus is operated has increased considerably, and the provision of satisfactory switching tubes has created numerous problems of design and fabrication. In the case of TR tubes designed for insertion in a waveguide, the tube itself is commonly made from a short section of wave guide having the ends closed off by irises or windows. Each iris or window is formed of glass or other insulating material and is dimensioned to be resonant at the frequency of operation of the apparatus. A gas fill is usually provided within the envelope and a tuning mechanism is also included to provide a means for setting or adjusting frequency of operation of the tube.

The requirement of operation at high powers presents a number of problems of fabrication, including the making and maintaining of the various hermetic seals between the elements of the device and the choice of suitable materials and components. Some of these have been overcome, but one serious. problem, namely sputtering, has continued to plague designers. Because the discharge takes place across the window or iris of the device, metallic material is often deposited upon the iris when the device is operated under high power condi-- tions. The voltage gradient across the window is such that the primary source of such sputtered metal is the metal surface immediately adjacent the window. The amount of metal deposited in some instances is sutncient to form a short circuit across the window, or the window may become opaque to microwave transmission or detuned from its designed frequency of operation.

It is therefore a primary object of the present invention to provide a switching device capable of high power operation at microwave frequencies.

It is another object of the present invention to provide a switching tube having a construction which eliminates the deleterious effects of sputtering.

These shorted or open circuits may be effective 2,990,568 Patented Aug. 18-, 1959 Another object of the invention is to provide a shield for preventing sputtered metal in a microwave switching device from being deposited on the iris 'or Window of the device. 2

In general, the present invention consists in a microwave switching device having a resonant window or windows whose electrical characteristics are relatively unaffected by sputtering. Each window is fabricated with a thickened or raised area or ridge at its outer edge adjacent the area of scaling to the metal. The ridge may extend completely around the periphery of. the window or may be limited to only the edges where the electric field is greatest. The window itself is constructed of dielectric material, glass or other ceramic being the preferred materials. The metal plate or frame in which the window to which the dielectric material is sealed may be of Kovar or other suitable metal having a coeflicient of expansion which approximates that of the material used in the-window itself. In the case of glass, the seal may be made directly to the Kovar and in the case of other ceramics, the edge of the ceramic is metallized and then brazed to the metal frame. Alternatively, the raised areas or ridges may be formed by punching such ridges into the metal of the frame and sealing glass over these metallic ridges.

Other objects, features and advantages of the invention will become apparent when the following description is considered in conjunction with the annexed drawings, in which:

Fig. 1 is a side elevation, partly in section, of a switching device in which the invention is incorporated;

Fig. 2 is a view of the switching device of Fig. 1, taken along the line 2-2, showing the window and frame; and

Fig. 3 is a sectional view of the window of Fig. 2 taken along the line 33.

The device shown in Fig. 1 is representative of the ATR type of switching tube in which the present invention may advantageously be incorporated. A tube of the TR type, insofar as the present invention is concerned, would be similar except that it would include other elements characteristic of TR type tubes, including windows at both ends of the device, a different tuning mechanism and a side rather than an end evacuation tip.

The tube illustrated includes a metallic envelope whic may be formed from a section of wave guide 12 or other suitably dimensioned member. A metallic plate 14 is hermetically sealed, for example by brazing, to the end of wave guide section 12. A generally rectangular opening is formed in the plate 14 to accommodate a frame 15 in which is sealed a. ceramic window 16 transparent to electromagnetic energy. The inside dimensions of the frame 15 which determine the window size, insofar as the passage of electromagnetic energy is concerned, are

chosen to provide resonance at the frequency of opera-- tion of the wave guide in which the switching device is to be used. The wall of the wave guide 12 opposite the window is closed off with a solid conductive plate 18.

It is the usual practice to tune ATR tubes to the desired frequency of operation by providing a diaphragm mounted within the wave guide section 12 adjacent back plate 18. Access to the diaphragm to. adjust the tuning is normally through an opening in plate 18, which may be closed after the tuning adjustment is made by sealing a tube therein. This tube is also utilized to evacuate the envelope, and is illustrated at 19 as it appears after tipping.

The frame 15 is punched with a stiffening ridge 20 which extends around its entire periphery. This ridge permits yielding of the frame 15 to reduce the likelihood of fracture of the seal between frame 15 and window 16. In some instances, to provide a relatively flush outside surface and to localize the brazing material subsequently used, a step 21 is formed about the entire window periphery. The step itself is metallized prior to the assembly of the window 16 into frame 15, metallization permitting a brazing operation to be used in sealing the window 16 to the frame 15. The depth of the step 21 matches the thickness of the frame 15 and thus provides a substantially flush joint between window and frame. In addition, the step on window 16 facilitates the proper placement of metallizing material and brazing to provide a clear opening through which electromagnetic energy may be transmitted when the device is in use in a wave guide. By this is meant that neither the metal of the frame 15 nor the metal of the sealing or brazing material interferes in any way with the dimensions of the window 16. This results in maintenance of operation at the resonant frequency as originally determined by proper design of. the opening in frame 15.

When the device thus far described is employed in a rectangular, wave guide system, the usual modes of energy transmission are such that the greatest potential difference exists across the shorter dimension of the aperture; i.e., between the longer edges of the frame 20. When the potential is sufliciently high a discharge occurs in the ionizable gas adjacent the window, the high current of the discharge causing liberation of metal from the frame 20 (and possibly plate 14) which may be deposited on the, window 16. In accordance with the present invention, the deposition of sputtered metal on the window is substantially eliminated by the provision on window 16 of upturned ridges or flanges 16a and 16b arranged parallel to each other and spaced apart preferably at least as great as the narrow dimension of the opening in frame 15. The ridges or flanges 16a and 16b extend into the envelope a short distance, in a direction preferably perpendicular to the plane of the window 16, and, in effect, provide shelves separating the long edges of the frame 15 from the window. Thus, such metal as is sputtered from these edges during high power operation of the tube lodges on the shelves and does not reach the area of the window 16 through which the wave energy is transmitted.

Althoughdeposition of metal on the window is substantially eliminated by flanges or ridges arranged along the longer edges of the window (across which the discharge normally occurs), if desired, the flanges or ridges may be extended in unbroken fashion around the entire periphery of the window, or at such other points about the window as the configuration of the electric fields and dominant area of breakdown may dictate.

In the fabrication of the illustrated preferred embodiment, that is, a window of ceramic having parallel flanges along the long edges, a unitary block of ceramic is milled to form the flanges 16a and 16b as well as the peripheral offset or step 21. After the milling operation, the peripheral step 21 is coated with a suspension of powdered metals and the entire block is placed in an oven for sinteriug. In the sintering operation, the metals become firmly bonded to the step 21. The ceramic window is then placed in the frame 15, and the assembly brazed, for example with copper, to form an hermetic seal between the window and the frame. After the window and frame are brazed together, the assembly is sealed to the metal plate 14 by a lower temperature brazing process utilizing a silver or silver-eutectic brazing material. In

some instances, the sealing of window to frame and frame to end plate may be carried out in a single operation in a reducing atmosphere, such as in a hydrogen furnace, the brazing material being placed in solid form adjacent the joints to be made. At the same time, or earlier in the process if desired, a flange 22 is brazed to the outside of section 12 to facilitate mounting of the device in a wave guide.

As previously noted, in fabricating TR tubes, a second window made in the same manner replaces the blank wall 18. In addition, a tuning mechanism more elaborate in nature than that used in ATR tubes may be included within the envelope 12. The tuning mechanism in neither case is of importance to the present invention, however, and will not be described in detail. The window which is substituted for blank wall 18 is identical in every respect to that formed by frame 15 and ceramic window 16. The evacuation tube is placed in a side wall and the keep-alive electrode, if used, is placed in the side wall opposite the evacuation tube.

In both the ATR and the TR tubes, a gas fill is included within the envelope 12 as previously noted. De' pending upon the requirements of ionization and de-ionization time, the gas fill may be any one or combination of a number of gases and vapors.

Although what has been described constitutes a preferred embodiment of the invention, other structures within the purview of the invention will suggest themselves immediately to those skilled in the art. By way of example as previously mentioned in this specification, the frame 15 may be formed with upstanding ridges of metal over which glass or ceramic may be subsequently sealed to provide the desired shielding function against sputtered metal. The invention should, therefore, be limited only by the spirit and scope of the appended claims.

What is claimed is:

1. In a microwave switching device including an envelope formed in part by a section of metallic wave guide, a diaphragm comprising a metallic frame defining a substantially rectangular window area sealed across an end of said wave guide, and a ceramic window structure seated in said area and sealed to said frame, said window structure having a flat portion lying in the plane of said diaphragm and including integral upstanding shelves extending along at least the long edges thereof and extending into said envelope in a direction substantially perpendicular to the plane of said flat portion for shielding said flat portion from metal sputtered during operation of the device.

2. A high frequency gaseous discharge device comprising a gas-filled envelope of hollow waveguide having a transverse metal frame hermetically sealed to an end thereof, said metal frame defining a central substantially rectangular window opening, and a unitary window structure formed of dielectric material seated in said opening and sealed to said frame, said window structure having a flat portion lying in the plane of said frame and includingv along the long edges thereof integral shielding shelves extending into said envelope beyond the plane of said flat portion perpendicularly to said flat portion, each shelf being coextensive with the long dimension of said opening.

References Cited in the file of this patent UNITED STATES PATENTS 

